1. Field of the Invention
This invention relates to a steering control system for a vehicle.
2. Description of the Related Art
Earlier patent applications proposed by the assignee, including those made public by Japanese Laid-Open Patent Application Nos. Hei 5(1993)-197,423 and Hei 9(1997)-221,054, teach vehicle steering control system technologies that provide steering angle assistance for keeping the vehicle on a desired course of travel along a lane of a road.
By providing fine steering assistance while preventing vehicle lane wandering, these technologies greatly reduce the burden on the driver, which tends to increase particularly during long, high-speed driving on an expressway or the like.
In view of the interference or interaction between this type of steering assistance control and driver steering, the control should preferably be effected to provide assistance through torque (steering force) rather than steering angle.
When the vehicle is equipped with both a lane holding steering torque assist control system of such type and a steering torque assist control system of the power steering, hereinafter referred to as the power steering torque assist, an issue arises regarding how to switch the mode of control between the two systems. If the assist torque changes abruptly at the time of switching, this will be disadvantageous in that, for example, the vehicle driver experiences annoyance upon sensing a change in steering torque assist.